systems in which conveyor belts are driven in a spiral path with the belt curving edgewise around a series of vertically spaced loops are commonly used to provide a great length of endless conveyor belt in a relatively small space. A particular advantage of such systems is that they can be used with high production volume operations while at the same time providing the long process dwell time required for certain processes, such as product warming or freezing.
One conveyor system of this type is disclosed in U.S. Pat. No. 4,741,430. This system uses a positive drive to propel the belt. The drive includes a cylindrical cage rotatable about a vertical axis and having a plurality of vertical driving bars spaced circumferentially around the cage. The bars have radial driving surfaces arranged in overlapping and abutting relationship with protrusions which are spaced along the radially inner edge of the belt in the spiral loops so as to positively drive the belt. Tension is maintained in the outside edge of the belt to hold the belt against the driving bars so that the driving action is accomplished. This tension is significantly lower than the tension the belt would experience in a straight line conveyor.
Positive drive spiral belt systems suffer certain drawbacks. It is difficult to match the speed of the multiple mechanical drives in the system. Engagement of the driving surface with the belt surface is difficult to initiate and maintain smoothly. The pitch of the belt changes as the belt enters and leaves the series of spiral loops. This changing pitch can not be compensated for by the drive mechanism. Thus, as belt pitch increases, driving contact is lost allowing the belt to become slack and migrate backward. A decrease in belt pitch causes the belt tension to increase resulting in belt surging, increased wear, fatigue failure and other damage to the system. Accordingly, although such a positive drive system offers the potential for very low conveyor belt tension, as a practical matter, a state of continuous, smooth, low tension operation is very difficult to achieve and maintain.
Another conveyor system of this type is disclosed in U.S. Pat. No. 3,348,659. In this system, the belt is not positively driven but frictionally driven. Tension is induced in the spiral belt so that there is frictional contact between the radially inner edge of the belt and a plurality of driving elements attached to a drive drum. This belt system is then frictionally driven by the driving elements which slidingly engage the radially inner edge of the spiral belt loops. The driving elements move faster than the belt and continuously slide past the belt edge to achieve a frictional drive.
This friction drive, while still achieving moderately low belt tensions, greatly reduces the need for, and problems associated with multiple drive speed matching. In such systems tension must be induced in the outer belt edge, thereby constricting the belt around the drive drum. This results in a contact force, F.sub.N, between the inside belt edge and the driving elements. This contact force times the coefficient of friction between the belt and driving elements results in the belt driving force.
This belt driving force must overcome the live load frictional force which is the belt, plus product weight times the coefficient of friction between the belt and belt support elements. Therefore, it can be shown that the required belt tension, T, is directly proportional to the belt to belt support friction f.sub.s or EQU T .alpha. f.sub.s
and is inversely proportional to the belt to belt drive element friction f.sub.D or EQU T .alpha. 1/f.sub.D
or combined ##EQU1##
Since conveyor belt life is a function of belt tension, it is desirable to achieve the lowest possible belt tension. As normally practiced, the belt drive elements and belt support elements are capped with a polyethylene, or ultra high molecular weight polyethylene face which acts as a wear surface between the metal or plastic conveyor belt and the metal conveyor elements.
In an effort to reduce belt tension by lowering f.sub.s, lubricated belt support plastic capping has been used. However, this has been known to contaminate the product the belt is transporting and over a period of time, combines with product fines to form an abrasive paste which actually increases f.sub.s.
Other various attempts have been made to increase f.sub.D by placing vertical ridges or grooves in the drive element capping which engage projections in the edge of the belt. While sometimes reducing belt tension, such measures normally result in undesirable belt vibration and product orientation problems. To reduce the belt vibration, the speed of the drive drum is lowered so that the relative speed between the drum and inside belt edge are more closely matched. This, however, results in a lowering of the drive force. As a result, belt tension must be increased, thus counteracting the initial benefits of the grooved wearstrips.